As with the NEX-7, most of my issues with Olympus' images are with the JPEG processing rather than the noise profile. You can see the noise reduction kick in aggressively between ISO 200 and ISO 400 (compare the text under the bill).
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Reds
The E-M5 handles bright, saturated colors very well in both raw and JPEG; even just the red channel of this image doesn't look blown out, thereby preserving detail in highlight areas, as I see with a lot of cameras. Olympus also defaults to a neutral color profile and generally delivers saturated but still quite accurate colors.
(1/200 sec, f11, ISO 200, AWB, spot metering, Panasonic 12-35mm f2.8 lens at 35mm)
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Sharpening
Here's an example of Olympus' aggressive default sharpening. It's also so broadly applied that it tries to force areas that perhaps aren't in focus into relief. This is both a blessing and a curse; photos that might otherwise have been a little too soft become usably sharp, but in-focus shots look oversharpened.
(1/250 sec, f6.3, ISO 200, AWB, spot metering, Panasonic 12-35mm f2.8 lens at 35mm)
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Highlight recovery
The camera's sensor does a pretty good job preserving detail in blown-out areas. Shadowed areas, however, can't be easily recovered without introducing a substantial amount of color noise, regardless of ISO sensitivity.
(1/100 sec, f6.3, ISO 200, AWB, spot metering, Panasonic 12-35mm f2.8 lens at 35mm)
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ISO 400
By ISO 400 I saw a lot of clipping and contouring in shadow areas, plus the occasional hot pixel from the image processing.
At ISO 400, you lose an incredible amount of detail in textures because of the noise suppression. Given that ISO 400 is just one stop up from the base of ISO 200, it makes using JPEG a problem.
With JPEGs, I wouldn't shoot above ISO 800; ISO 200 if you're really picky about detail. But you can probably go as high as ISO 1600 for decently lit shots -- there's too much clipping in the shadows on dark images to recommend this camera for midrange ISO sensitivities, though.
(1/60 sec, f6, ISO 1600, AWB, spot metering, Olympus 12-50mm f3.5-5.6 lens at 43mm)
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ISO 1600, no noise reduction
The E-M5 has generally fine-grained noise.
(1/60 sec, f6, ISO 1600, AWB, spot metering, Olympus 12-50mm f3.5-5.6 lens at 43mm)
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ISO 3200
By ISO 3200, I couldn't get any images, raw or JPEG, that I'd consider usable at full size. Viewed at 50 percent or smaller, though, they look OK.
(1/60 sec, f8, ISO 1600, AWB, spot metering, Olympus 12-50mm f3.5-5.6 lens at 43mm)
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ISO 6400
ISO 6400 shots show artifacts even when viewed or scaled to 50 percent; they're OK at about a third of the original size.
(1/40 sec, f6.3, ISO 3200, AWB, spot metering, Olympus 12-50mm f3.5-5.6 lens at 50mm)
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Video
Though shot at dusk, this video grab gives a fairly representative idea of what the E-M5's video tonality looks like, including the somewhat cool white balance.
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Video
I like the general visual quality of the E-M5's night video, but it suffers from the same problems as the stills: you can see the noise suppression artifacts.